When Your FM Interferes With… Your Own FM (And You Don’t Even Realize It)

You have a 10 kW station. The tower looks imposing, with the antenna at the top, everything suggesting the signal should cover the region well. However, after driving just a few kilometers, in certain areas something strange happens.

Your FM seems to lose channel definition. In some places the audio hisses, scratches, or briefly cuts out, something almost unimaginable for a station that should deliver a solid signal for at least 30 km. And the phenomenon repeats every day in the same locations.

The sound seems to become more muffled and weaker, as if it had lost presence and clarity.

One of the first things to check is whether the tower ended up being surrounded by buildings.

Imagine a light bulb surrounded by mirrors and translucent glass surfaces. Part of the light follows its normal path, but another part reflects, returns, and mixes in an altered way. The result is distorted lighting. Something similar happens with radio signals.

There is no spontaneous reinforcement of the signal in these reflections. In many cases the opposite happens: parts of the signal reach the receiver with different delays and interfere with each other. In certain situations partial cancellation may even occur, meaning the station’s own signal ends up interfering with itself.

Multipath

This phenomenon is known in broadcast engineering as multipath. It occurs when the FM signal travels along multiple paths, reflecting off buildings, hills, metallic structures, or other surfaces before reaching the receiver. Each of these paths has a small delay, which causes interference between the copies of the same signal.

The practical result can be

Loss of audio definition

A sensation of muffled sound

Momentary hissing or scratching

Stereo fluctuation

Noticeable distortion in certain parts of the city

Fading

Below I present an article by one of the leading engineers in the field, with extensive experience in studios and broadcasting in the United States. The text also includes the perspective of Jorge Faria from AudioTX, responsible for one of the audio processors that stands out both for its quality and its excellent cost benefit ratio.

Negative effects of multipath on the FM stereo signal

Multipath is one of the main enemies of FM stereo in mobile or urban reception.

Multipath occurs when the FM signal reaches the receiver through multiple paths, both direct and reflected by buildings, mountains, or other obstacles. In FM stereo this phenomenon can cause distortion, noise, and instability in the stereo image, severely degrading the listening experience.

How multipath affects FM stereo

Phase and amplitude variation

Reflections arrive with different delays, causing constructive and destructive interference. The L minus R channel, which operates on the 38 kHz subcarrier, is more sensitive to phase changes. This compromises stereo separation and causes distortion.

Stereo image oscillation

The listener perceives the sound moving between left and right or losing its center position. In extreme cases the stereo separation may temporarily invert.

Increased noise and hiss

Stereo reception amplifies noise in the L minus R subcarrier. In areas with strong multipath the hiss becomes more noticeable and constant.

Flanger or phasing effect

Microsecond delays in reflected signals create interference similar to musical effects pedals, producing variations in tone and frequency.

Loss of voice clarity

In news radio, multipath can make speech less intelligible because noise and phase variations mask consonants and fine details.

Why mono suffers less

In mono the receiver ignores the L minus R channel and the 19 kHz pilot. The audio is reconstructed only from L plus R, which is more robust and less affected by phase variations, resulting in a more stable and cleaner sound.

Scenarios where multipath is more critical

Urban centers with building canyonsHighways surrounded by hills or rocky wallsVehicles moving quickly, with constant variation in the signal path

Conclusion

Multipath is one of the main enemies of FM stereo in mobile or urban reception. The most effective technical solution, when clarity is the priority, is to operate in FM mono or apply processing and transmission techniques that increase the robustness of the L plus R channel.

Phase and amplitude variation in FM

Summary

In FM transmission, phase and amplitude variation is generally related to propagation problems such as multipath. Although FM is a frequency modulation system and not an amplitude modulation system, reflections and interference can produce apparent phase and amplitude changes in the received signal, affecting audio quality.

What phase variation means in FM

Phase is the relative position of a wave in relation to a reference point.

In FM stereo, especially in the L minus R channel operating on the 38 kHz subcarrier, rapid phase changes can cause

Instability in the stereo imageLoss of channel separationFlanger or phasing effects in the sound

Multipath is the main cause because reflected signals arrive with different delays and alter the phase of the composite signal.

What amplitude variation means in FM

Although the information in FM is carried in the frequency of the carrier and not in amplitude, the receiver is still sensitive to amplitude drops or peaks caused by interference.

These variations are perceived as

Momentary loss of volumeNoise or flickering in the audioBrief interruptions during mobile reception

Relationship with stereo audio

The L plus R channel from 0 to 15 kHz is more robust against phase and amplitude variations. The L minus R channel at 38 kHz suffers more because it is carried on a subcarrier and depends on phase synchronization to be decoded correctly. When the phase changes rapidly, the receiver may decode incorrectly, causing the stereo image to wander or even invert channels.

How to mitigate

Transmit in mono in critical areas, eliminating the L minus R channelReduce stereo width in the processor such as in Stereo Tool FMAdjust and maintain antennas to reduce multipath zonesUse reception diversity with dual antennas in professional systems

In FM audio processing, phase and amplitude variations have a direct impact on sound quality and modulation consistency. Although FM modulation encodes information in the frequency of the carrier, the processing and multiplex stages must deal with the audio spectrum and subcarriers including L plus R, L minus R, pilot, and RDS, which can be affected by phase and amplitude distortions.

What phase variation means in the context of FM audio

Definition: change in the temporal alignment between different components of the signal.

Impact on FM stereo

The L minus R channel on the 38 kHz subcarrier depends on precise phase alignment for stereo reconstruction. Misaligned phase causes loss of stereo separation, a floating stereo image, and even channel inversion. Misalignment between bands in multiband processing can generate hollow sounding audio or harsh highs.

Article written by Jorge Faria 06 February 2025

Reference sources

Effect of Multipath and Stereo Separation in FM TransmissionSmith S 1999 The Scientist and Engineer’s Guide to Digital Signal Processing California Technical Publishing

ITU R BS 412 9 International Telecommunication UnionInternational standard on technical requirements for FM transmission including considerations on stereo separation and bandwidth.